Following the introduction of the varicella vaccine in the U.S. in 1995, the incidence of wild-type varicella declined dramatically in vaccinated populations. However, each year a small proportion of vaccinated persons experiences breakthrough varicella, which usually manifests as a mild, atypical presentation that can be difficult to diagnose. In addition, herpes zoster after vaccination is not uncommon, and vaccine-type varicella virus has been isolated from some cases. In response to Objective Two of PA-04116 (conduct applied research for varicella and/or herpes zoster), we will study breakthrough varicella and post-vaccination herpes zoster. Our study objectives are (1) To measure the reliability of provider diagnosis of breakthrough varicella by comparing provider diagnosis with laboratory PCR results; (2) To examine cases of herpes zoster after varicella vaccination, specifically identifying the virus strains (wild-type versus vaccine-type) and calculating incidence rates of vaccine- related herpes zoster. These investigations will be conducted in the population of Kaiser Permanente Northwest, a closed-panel health maintenance organization with the ability to identify and enroll cases as they present to the medical system. Laboratory isolation of varicella virus and identification of virus strain through PCR will be conducted at the Centers for Disease Control varicella laboratory. KPNW researchers and the CDC laboratory have worked together previously on similar studies. This proposed study offers the opportunity to collect population-based information on important sequelae of varicella vaccination.